All oils and fats have a resistance to oxidation which depends on the degree of saturation, natural or added antioxidants, prooxidants, or prior abuse. Oxidation is slow until this resistance is overcome, at which point oxidation accelerates and becomes very rapid. The length of time before this rapid acceleration of oxidation is the measure of the resistance to oxidation and is commonly referred to as the ?induction period.? In this method for determining the induction period, a stream of purified air is passed through a test sample of oil or fat which is held in a thermostated bath. The effluent air from the oil or fat test sample is then bubbled through a vessel containing deionized water. The conductivity of the water is continually monitored. The effluent air contains volatile organic acids, swept from the oxidizing oil, that increase the conductivity of the water as oxidation proceeds. Formic acid is the predominant organic acid formed (see References, 1). The conductivity of the water is monitored by a computer or strip chart recorder. The Oil Stability Index (OSI) is defined as the point of maximum change of the rate of oxidation, or mathematically as the maximum of the second derivative of the conductivity with respect to time (see Fig. 1). This time-based end point may be determined by a computer that can calculate the maximum of the second derivative with respect to time, or by a slope-change algorithm, which is similar to detecting the onset of peaks for integration of GLC chromatograms. The end point may be approximated by using other methods. One commonly used approximation is a graphic method in which tangents are drawn manually (see Fig. 2). The OSI may be run at temperatures of 100, 110, 120, 130, and 140°C. Because by its nature this analysis has this temperature flexibility, all OSI results should specify the OSI time, with the analysis temperature reported immediately after (for example, ?OSI 11.7 hours at 110°C?).